Cast iron with high creep resistance and method for making same



Oct. 15, 1957 R. D. SCHELLENG ET AL CAST IRON WITH HIGH CREEP RESISTANCE AND METHOD FOR MAKING SAME Filed Nov. 14, 1955 FIG. 1

ROBERT DOUGLAS SCHELLENG JOHN TRIMBLE EASH 'INVENTORS ATTORNEY United States Patent CAST IRON WITH HIGH CREEP RESISTANCE AND METHOD FOR MAKING SAME Application November 14, 1955, Serial No. 546,742 4 Claims. (Cl. 75--125) The present invention relates spheroidal graphite and having sition and, more particularly, to such cast irons characterized by greatly improved mechanical properties at elevated temperatures, including improved creep resist ance. 1

It is well known that ordinary flake graphite gray cast irons, particularly cast irons having ferritic matrices, have very poor mechanical properties at elevated temperatures. In particular, the creep strength of ferritic flake graphite cast irons has been so poor that the use of such materials at elevated temperatures has heretofore been greatly restricted. As an example, it may be noted that the creep strength for ordinary flake graphite cast iron approximates 8000 pounds per square inch for a creep rate of about 1% in 10,000 hours at 840 F. When cast irons containing graphite in the much less harmful form of spheroids became available to the art through the disclosure of Millis et al. in U. S. Patent No. 2,485,760, a sudden and dramatic advance in the strength of materialsnominally having the composition of ordinary gray cast iron was made. This advance in the properties of such materials was so striking that it became possible to employ materials nominally having a cast iron composition in applications in which it had not previously been possible to use cast iron. The outstanding advance in'the properties of cast iron made possible by the said Millis et al. invention also suggested that it to ca-st irons containing a special limited compomight now be possible to employ materials having a cast iron composition in service at elevated temperatures of about 600 F. to about 1000 that many of thenew cast irons, particularly those having a ferritic matrix, having greatly improved room temperature strength were not appreciably stronger in creep resistance at elevated temperatures than the flake graphite cast irons of the prior art. Accordingly, it became apparent that control of the graphite in cast iron to a spheroidal form was not by itself sufficient to secure a product having an improved resistance to creep.

"We have now discovered a ferritic magnesium-containmg cast iron containing graphite in the spheroidal form and having greatly improved mechanical properties at elevated temperatures and, in particular, improved creep resistance. V

, It is an object of the present invention to provide a new ferritic cast iron having improved creep resistance.

Another object of the invention is to provide a method for heat treating a special magnesium-containing cast' iron to provide improved creep resistance therein.

Other objects and advantages of the invention will" description, taken in conjunction with the accompanying drawing, in which;

Figure'l is a reproduction of aphotomicrograph taken become apparent from the following at 500 diameters depicting the structure of a casting outside the scope of the present invention after an anneal at about 1750 F. for about ,16 hours and after subsequent exposure to a creep test at 800 F.;

"Fig. 2 is a reproductionof a photomicrographtaken F. However, it was found in accordance with the those skilled in the art will readily-understand. In carat 500 diameters depicting the structure of a casting made present invention after an anneal at about 1750 'F. for about 16 hours and after subsequent exposure to a creep test at 800 F.; and

Fig. 3 is a reproduction of a photomicrograph taken at 1000 diameters depicting the structure of the same casting shown in Fig. 2.

7 Generally speaking, the present invention contemplates a special ferritic cast iron containing about 3% to 4% carbon, about 2% to 6% silicon, about 0.35% to 1.1% copper, about 0.5% to 1.25% manganese, about 0.05% to 0.1% phosphorus, about 0.75% to about 1.5% nickel, magnesium in a small amount up to about 0.1%, e. g., about 0.02% to about 0.1%, to effect the occurrence of a high proportion, e. g., about 90% or more, of the graphite present in the cast iron in a spheroidal form,

1 and the balance essentially iron including small amounts of minor constituents and impurities commonly present in ferritic magnesium-containing cast iron'. The iron content is usually at least about 87% of the composition. Molybdenum in smallamounts up to about 1% is beneficial from the standpoint of improving the creep resistance of the cast irons. It is preferred that the special position produced in accordance with the invention contain about 0.6% copper, about 0.8% manganese, about 0.08% phosphorus, about 1% nickel, about 3.5% carbon, about 2.5% silicon and about 0.04% to about 0.07% magnesium to eifect the occurrence of a high proportion of the graphite present in the cast iron in a spheroidal form, with the balance essentially iron and the iron content being at least about 90% of the composition.

It has been found that an annealing heat treatment improves the creep resistance of alloys produced in accordance with the invention. Such annealing treatment comprises a heating to a temperature of about 1650 F. to about 1800 F., preferably about 1700 F. to about 1800 F., holding Within said temperature range for about'Z to about 20 hours, followed by slow cooling, e. g., furnace cooling, to a temperature below the alphagamma transformation temperature for the alloy, e. 'g., about 1275 F. The slow cooling carried out in con junction with the annealing operation is conducted at a rate not exceeding about 10 F. per hour. Of course, the slow cooling may be carried out at any slower practical rate depending upon the equipment employed, etc., as

ferritic cast iron comrying out the annealing treatment, it is desirable that the resulting ferritic grain size be no finer than about 8 on the A. S. T. M. Standard E 89-52 grain size chart.

- More improved creep properties are developed in castings having a relatively coarse ferritic grain size in the range of about 2 to 6, e. g., about 4, on the A. S. T. M.

grain size chart. As previously stated, cast irons contemplated in accordance with the invention exhibit improved creep resistance after the annealing treatment is conducted. On the other hand, cast irons containing copper in amounts lower than the minimum amount of copper required in accordance with the invention actually exhibit a reduced creep resistance after having been sub.- jected to an annealing treatment in accordance; with the invention. 8 Thus, in one instance, an annealed cast.

iron containing only 0.15% copperexhibited a creep strength of only 7500 pounds per square inch for a creep rate of 1% in 10,000 hours'at 800 F. On the other hand, a similar cast iron .within the present invention and containing 0.56% copper. exhibited a creep strength of 2 5,000 pounds per square inch for a creep rate of 1% in 10,000 hours at 800 F.- after the same annealing treatment. A characteristic microstructural effect is observed in cast irons having improved heat resistance asv contemplated in accordance with the invention after a;

Patented Oct. 15, 1957 heat treatmentsuch as that described hereinbefore, to wit, a finely divided discrete phase, which is dispersed throughout the structure of the special cast iron. This dispersed phase is illustrated in- Figs. 2 and?! of theaccompanying drawing. These figures-are reproductions ofphotornicrographs takenat 500 diameters and 1000- diameters, respectively, showing the structure ofa' castiron con-- taining about 0.56% copper produced in accordance-with thepresent invention as described in Example I after an anneal at 1750 F. and after a creep test at 800 F; and 20,000 pounds per square inch for about 6066 hours. The discrete phase appearing as very many small, well distributed globules in the ferrite grains and in the grain boundaries visible in.these figures. By way of contrast, Fig. 1, whichis a reproduction of a photomicrograph taken at about 500 diameters of an otherwise similar cast iron containing only about 0.15% copper and outside the scope of the present invention, shows only a veryfew small inclusions. Fig. 1 was taken from a cast iron specimen which also had been annealed at 1750 F. and which had been exposed to a creep test at 800 F. and 15,000 pounds per square inch for about 975 hours.

It has been established that the various components of the composition must be maintained within the ranges given hereinbefore in order to provide improved properties as contemplated in accordance with the invention. The effect of copper is particularly important and the copper contentis not less than about 0.35% as otherwise the improved creep resistance characterizing cast iron in accordance with the invention is not obtained. On the other hand, the copper content should not exceed about 1.1% as otherwise the mechanical properties of the cast iron, and in particular the ductility, and the spheroidal graphite structure are detrimentally affected. The man'- ganese content of the cast iron in conjunction with the copper likewise contributes to the creep resitsance of the cast iron. The manganese is at least about 0.5% to realize the beneficial effect of manganese upon the creep strength of the alloy but does not exceed 1.25% as otherwise pearlite in the structure becomes too stable to decompose readily to ferrite on heating as contemplated in accordance with the present invention. The phos-' phorus content of cast irons produced in accordance withthe invention is likewise important and the phosphorus is maintained at not less than about 0.05% as otherwise the creep strength of the irons is diminshed. The phosphorus content does not exceed about 0.1% to avoid an undesirable decrease in the ductility of the cast iron. The nickel content of the special cast iron is likewise important and is maintained in the range of about 0.75% to about 1.5% in order to cooperate with the copper content of the cast iron in achieving high creep strength after an anneal as contemplated by the invention to produce a ferritic structure whereby stability is imparted to the cast iron structure when it is exposed to heat. The carbon content is maintained within the range of about 3% to about 4% in order that the castings will be free from primary carbide. The silicon content is likewise important and should be at least about 2.25% in order to provide oxidation resistance as otherwise the ductility of the iron is reduced.

In order to give those skilled in the art a better understanding of the invention, the following illustrative examples are given:

Example I thereafter given a graphitizing inoculation with ferrosi1i con. The melt was then cast into castings which contained the foregoing amounts of carbon, manganese, phosphorus and copper and which contained about 2.5% silicon, about 1% nickel, about 0.072% magnesium, and had graphite in a spheroidal form. Certain of the castings thus produced were annealed at 1650 F. for 2 hours. These castings were found to have an A. S. T. M. grain size of about 6 to 8. Certain other castings were annealed at 1750 F. for 16 hours. These castings were found to have an A. S. T. M. grain size of about 4 to 6. Annealed castings were then subjected to creep test at 800 F. with the following results after 2000 hours:

Stress in Creep Rate, Annealing Temperature Creep Test, percent in p. s. i. 10,000 hours 20, 000 1.05 15, 000 0.13 15, 900 0.11 1,750 20, 000 0.27 & Example ll Another melt of cast iron was made in accordance with the procedure. set forth in Example I and was cast to produce castings containing about 3.6% carbon, about 2.5% silicon, about 0.8% manganese, about 1.2% nickel, about 0.087% phosphorus, about 0.07% magnesium, and about 0.6% copper. Two castings produced from the cast iron were annealed at 1650 F. for 2 hours and were then found to have an A. S. T. M. grain size of about 6 to 8. The castings were subjected to a creep test in the annealed condition at'a temperature of 800 F. at a stress of 20,000 pounds per square inch. These castings exhibited, respectively, a creep rate of 0.80% and 0.90% in 10,000 hours in the creep test.

The results of the creep tests demonstrate that the cordance with the invention is very substantial as compared with prior art cast irons of the same alloy content and is comparable to cast steels of low alloy content. As an example, annealed low alloy cast steel castings have heretofore been determined to have a creep rate per 10,000 hours under a load of about 21,000 pounds per square inch at 800 F.

' Castings produced in accordance with the invention have substantial tensile strength in conjunction with high ductility in the ferritic condition. The room temperature properties of castings produced in accordance with the present invention will generally have mechanical properties within the following ranges in the annealed condition:

Yield strength Tensile strength Elongation 45,000 to 55,000 p. s. 1'. 65,000 to 80,000 p. s. i. 25% to 15% As indicate-d hereinbefore, the special cast iron contemplated in accordance with the invention can be produced in the usual] melting equipment employed for melting cast iron, e. g., induction furnace, electric furnace, the cupola, etc.

Cast iron castings produced in accordance with the invention and having enhanced creep strength are useful in many applications, including valves, furnace parts, melting and heat treating pots, manifolds, ingot molds, grate bars and in other applications in which the mechanical components are subjected to the deleterious effects of elevated temperatures.

While the theory underlying the improvement in creep strength which characterizes the special cast irons produced in accordance with the present invention is not it can be pointed out that it is unusual to find an association between grain coarsening and improvement in creep strength in a ferritic material in the temperature range around 800 F. Accordingly, it is believed that the annealing treatment conducted upon the special copper-containing spheroidal graphite cast iron as contemplated by the present invention cooperates therewith to bring about a redistribution of phases therein which, in conjunction with the coarse ferritic structure, contributes importantly to bring about an improvement in creep strength in the castings.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be'resorted to without departing from the spirit and scope of the invention, as those skilled in the art will readily understood. Such modifications and variations arecotnsidered to be within the purview and scope of the invention and appended claims.

We claim:

1. As a new article of manufacture, a creep-resistant ferritic spheroidal graphite cast iron casting characterized by the presence of a discrete phase in the form of very many small particles distributed through the ferritic matrix and containing about 3% to 4% carbon, about 2.25% to 6% silicon, about 0.35% to 1.1% copper, about 0.5% to 1.25% manganese, about 0.05% to 0.1% phosphorus, about 0.75% to 1.5% nickel, up to about 1% molybdenum, magnesium in a small but effective amount up to about 0.1% to efiect the occurrence of graphite in a speroidal form, and the balance essentially iron.

2. As a new article of manufacture, a creep-resistant ferritic spheroidal graphite cast iron casting characterized by the presence of a discrete phase in the form of very many small particles distributed through the ferritic matrix and containing about 3.5% carbon, about 2.5% silicon, about 0.6% copper, about 0.8% manganese, about 0.08% phosphorus, about 1% nickel, up to about 1% molybdenum, about 0.04% to about 0.07% magnesium to efiect the occurrence of graphite in a spheroidal form, and th alan e s ent a y ir n.

produce in said casting a 3. The method for producing a ferritic cast iron having improved creep resistance which comprises annealing at a temperature of about 1650 to about 1800" F. a casting containing about 3% to 4% carbon, about 2.25% to 6% silicon, about 0.35% to 1.1% copper, about 0.5% to 1.25% manganese, about 0.05% to 0.1% phosphorus, about 0.75% to 1.5% nickel, up to about 1% molybdenum, magnesium in a small but effective amount up to about 0.1% to effect the occurrence of graphite in a sphe roidal form, and the balance essentially iron, to produce in said casting a ferritic grain structure and to impart improved creep resistance to said casting in the service temperature range of about 600 to 1000 F.

4. The method for producing a ferritic cast iron having improved creep resistance which comprises annealing for at least about 2 hours at a temperature of about 1700 to about 1800 F. a casting containing about 3.5% carbon, about 2.5 silicon, about 0.6% copper, about 0.8% manganese, about 0.08% phosphorus, about 1% nickel, up to about 1% molybdenum, about 0.04% to about 0.07% magnesium to eifect the occurrence of graphite in a spheroidal form, and the balance essentially iron, to ferritic grain structure and to impart improved creep resistance to said casting in the service temperature range of about 600 to 1000 F. 

1. AS A NEW ARTICLE OF MANUFACTURE, A CREEP-RESISTANT FERITIC SPHEROIDAL GRAPHITE CAST IRON CASTING CHARACTERIZED BY THE PRESENCE OF A DISCRETE PHASE IN THE FORM OF VERY MANY SMALL PARTICLES DISTRIBUTED THROUGH THE FERITIC MATRIX AND CONTAINING ABOUT 3% TO 4% CARBON, ABOUT 2.25% TO 6% SILICON, ABOUT 0.35% TO 1.1% COPPER, ABOUT 0.5% TO 1.25% MANGANESE, ABOUT 0.05% TO 0.1% PHOSPHORUS, ABOUT 0.75% TO 1.5% NICKEL, UP TO ABOUT 1% MOLYBDENUM, MAGNESIUM IN A SMALL BUT EFFECTIVE AMOUNT UP TO ABOUT 0.1% TO EFFECT THE OCCURRENCE OF GRAPHITE IN A SPEROIDAL FORM, AND THE BALANCE ESSENTIALLY IRON. 